1. Field of the invention
The present invention relates to an improvement of a magnetic recording medium comprising a nonmagnetic support and a magnetic recording layer.
2. Description of prior arts
A magnetic recording medium (also referred to hereinafter as a magnetic tape) such as an audio-tape, a video-tape, or a recording medium employed in a computer system, basically comprises a nonmagnetic support and a magnetic recording layer provided on the support. The magnetic recording layer comprises a ferromagnetic powder such as a needle crystalline powder of .gamma.-Fe.sub.2 O.sub.3, Co-containing ferromagnetic iron oxide, or CrO.sub.2 dispersed in a binder. Recently, a demand for a higher density recording system has increased, and hence studies of a magnetic recording medium using a ferromagnetic metal powder in place of the conventional oxide-type ferromagnetic powder have been made.
The ferromagnetic metal powder has been employed as a ferromagnetic powder of a recording medium of a computer system. The ferromagnetic metal powder is high in a coercive force (Hc) and a residual flux density (Br) and moreover is stable in a wide temperature range. For these reasons, the ferromagnetic metal powder has been also paid attention for the use in such magnetic recording media as a video-tape or an audio-tape. The ferro-magnetic metal powder is known to afford recording of a higher density, as the powder is more minimized.
While the ferromagnetic metal powder has excellent characteristics, there is a drawback that the ferromagnetic metal powder is low in the hardness. In more detail, a magnetic recording layer using the ferromagnetic metal powder shows poor running property, that is, the recording layer is easily scratched or the metal powder is easily dropped off from the layer in the course of running in contact with a magnetic head. These troubles cause the so-called drop-outs. Further, the dropped metal powder is apt to adhere to the surface of the magnetic head to cause clogging on the head. Furthermore, if a magnetic layer has a poor running property, the magnetic layer shows only a short still life in the still mode in which a still video image is continuously reproduced.
It is known that an abrasive (i.e., hard particles) such as corundum, silicon carbide or chromium oxide is incorporated into a magnetic recording layer to improve the running property of the recording layer under the conditions to allow maintaining the electromagnetic conversion characteristics of the magnetic recording medium using a ferromagnetic metal alloy powder. Generally, however, the effect of incorporation of an abrasive is observed only where the abrasive is incorporated in a large amount. But, a magnetic recording layer containing a large amount of an abrasive likely causes extreme abrasion of a magnetic head employed in contact with the recording layer. Further, a large amount of an abrasive is unfavorable to the electromagnetic conversion characteristics of the recording medium. Furthermore, the incorporated abrasive is apt to be easily dropped off to adhere to the surface of the magnetic head.
For the above-described reasons, the conventional measure for improving the running property of the magnetic recording layer is not thought to be satisfactory, in view of the resulting characteristics of the magnetic recording medium as well as unfavorable effect given to a magnetic head.